In these last years the most wide spread technology for the construction of bridges made up of reinforced prestressed concrete is the technology that utilizes prefabricated segments or quoins to make up the bridge. This technology consists essentially in the prefabrication of the bridge segments, some meters long, which are installed in succession side-by-side on special supporting and launching means. Prestressed wires are then inserted and stretched to engage the quoins solidly against each other and thus render the pertinent bridge length self-bearing. The support means are then recovered and reutilized. These means are constituted of metallic carpentry and also provided with stays.
This technology is now receiving a new impulse from the use of prestressed wires disposed outwardly of the concrete section of the segments or quoins.
Among the means used to sustain the segments or quoins prior to the prestressing operation, only two systems are particularly interesting: The first utilizes a metallic beam with stayed wires for supporting the segments close to each other prior to the prestressing in order to form a bay; and the second utilizes a temporary stay for each segment or quoin until a complete beam is made up and, after the prestressing of the plank, the temporary stays are removed and the procedure is repeated. A feature that is common to the two systems is the use of a metallic pole from which two sets of stays branch off, one being oriented towards the piers to be reached by the new bay under formation, the other towards the pier already surpassed and anchored thereto for the necessary balancing of the forces arising from the stays which support the bay under construction.
Referring to the first system, it is useful to point out that if the height of the pole is to be kept within acceptable limits of economy and practicality--especially during the self-launching of the apparatus from one bay to the other--the minimum inclination of the stays must be about 25.degree., since a lesser inclination would cause a greater force in the stays and a greater horizontal thrust in the metallic beam from which the prefabricated segments or quoins hang.
The vertical reaction on the pole is approximately twice as much the weight borne by the metallic beam, the stays inclination towards the pier to be reached being substantially equal to that of the stays making up the anchorage towards the pier already surpassed, and the reactions of both the opposite sets of stays being discharged onto the pole.
These considerations--which are valid also for the above mentioned second system--limit the field of application of the two systems to spans of about 50 meters, and also require to oversize supporting apparatuses for the bridge since, in general, the ratio between the steady loads and the overloads is greater than 1.